A 24 × 12 ft deck in 5/4 × 6 boards with ⅛ in gaps takes 26 rows and 52 boards: 13 of 16 ft, 26 of 12 ft and 13 of 8 ft, laid so every butt joint sits on a joist and no two neighbouring rows break on the same one. Add 10% spare and you buy 58 boards and about 1,040 deck screws.
How deck boards are counted
Boards laid straight run across the joists, so the number of rows comes from the deck width and the board pitch (board width plus gap). The last row is ripped to fit.
Rows = (deck width + gap) ÷ (board width + gap), rounded up
Each row is then as long as the deck. When it is longer than your longest board, the calculator splits it at joists: it tries every way to cut the row into stock lengths, keeps the ways with the fewest butt joints, and among those picks the one with the least offcut. If a split would line up with the joint in the row before, it moves the joint to the next joist unless that wastes more than about a foot. Any offcut long enough for a shorter piece (the corners of a diagonal deck, the short side of a picture frame) is cut from it before a new board is bought.
Worked example: 24 × 12 ft
With 5½ in boards and ⅛ in gaps the pitch is 5⅝ in, and (144 + ⅛) ÷ 5⅝ = 25.6, so 26 rows, the last one ripped to 3⅜ in. A 24 ft row cannot be one board, so it needs one joint. On joists 16 in apart the joint can fall at 8, 12 or 16 ft; 12 + 12 and 8 + 16 both use the stock exactly, so the rows alternate between them: 13 boards of 16 ft, 26 of 12 ft and 13 of 8 ft, 26 butt joints and no offcut. Ten percent spare adds 6 boards.
If your yard only stocks composite lengths of 12, 16 and 20 ft (the Trex Enhance range), the same deck becomes 52 boards of 12 ft, every row breaking at 12 ft. The drawing shows why that looks worse: the joints form a straight seam, so staggering is worth an extra length in the rack.
Screws or hidden clips
The IRC (R507.7) and the American Wood Council's DCA 6 guide both call for two 8d threaded nails or two No. 8 screws per board at every joist. The calculator counts every place a board crosses a joist: 26 rows × 19 joists is 494, plus the second board at each of the 26 joints, 520 crossings in all, 1,040 screws, about 3.6 per square foot.
Hidden clips hold the edge of two boards at once, so they are counted once per crossing plus the starter edge: 540 clips, or 6 boxes of Trex Hideaway clips (90 to a box, which Trex rates at 50 sq ft on joists 16 in on center). Picture-frame boards are square-edged and still get face screws.
Board widths and joist spacing
| Board | Actual size | Max joist spacing, straight | At 45° |
|---|---|---|---|
| 5/4 × 6 wood | 1 × 5½ in | 16 in | 12 in |
| 2 × 6 wood | 1½ × 5½ in | 24 in | 24 in |
| Trex composite | 0.94 × 5.5 in | 16 in | 12 in |
Wood sizes are ProWood's actual dimensions and the spacings are IRC Table R507.7 for boards over three or more joists; the Trex row is from Trex's joist spacing guide. Set the joist spacing on the calculator to match; the code check turns orange when the boards would span too far.
Diagonal decks and picture frames
Diagonal boards need closer joists and more material. The same 24 × 12 ft deck at 45° on joists 12 in apart takes 55 rows and 48 boards, with about 59 ft of offcuts even after the short corner pieces are cut from the long offcuts. The rows are shortest at the corners, so most of the order is in long boards.
A picture-frame border runs one board around the edge, mitered at the corners, and the field boards stop against it. The two short sides of the frame run along the joists, so they need blocking or a doubled joist underneath; the field boards also end on blocking beside the frame. The calculator counts the frame as four extra pieces from your stock lengths.
A deck material list, not just boards
This page covers the walking surface. For joists, beam, posts, footings, ledger bolts and hangers use the deck calculator, which runs the same board layout inside a full framing plan. If you are still choosing between 12 and 16 in spacing, the deck joist span calculator shows what each joist size can span at each.
What to ask next
- How much stain the new boards need, with railing and steps: deck stain calculator
- What the whole deck costs in treated pine, cedar or composite: deck cost calculator
- How to step down from the deck: stair calculator
Use it as a decking calculator: enter the deck size and board width and it returns the boards, screws and waste to buy.